Carbon dioxide (CO2) is an abundant, inexpensive, and non-toxic renewable resource for the production of value-added chemicals and materials. Chemical fixation of carbon dioxide is an important research field of green chemistry. Alternating copolymerization of carbon dioxide-based polycarbonate is one of its most important applications. This polymer not only has excellent barrier properties to oxygen and water, but also has excellent biocompatibility and biodegradability. Polycarbonates can be used as engineering plastics, non-polluting materials, disposable medical and food packaging, adhesives and composite materials.
One of the promising approaches to harness the potential of CO2 is to copolymerize of CO2 with epoxides to produce polycarbonates. The most successful CO2-epoxide copolymerization systems are based on transition metal Cr(III), Co(III) or Zn(II) complexes with Schiff base ligands. In the case of copolymerization of CO2 and propylene oxide (PO), totally alternated poly(propylene carbonate) (PPC) with molar mass up to 300,000 g mol−1 could be prepared using a recyclable catalyst (salen) Co(III) (S. Sujith, et. al, Angew. Chem. Int. Ed., 2008, 47, 7306). To improve the thermal and mechanical properties of most investigated polycarbonates, (PPC) and poly(cyclohexenecarbonate) (PCHC), or endow degradable properties to other polymeric materials, incorporation of two or more other blocks into the polycarbonates to form block copolymers is indispensable. One strategy is the copolymerization of CO2 with other epoxides which could afford polycarbonate block copolymers. Through sequential addition of functionalized cyclohexene monomer, Coates, et. al. synthesized a multiblock polycyclohexene carbonate with different functional substituents at the cyclohexene ring with Zn(II) diiminate as catalyst (J. G. Kim, et. al., Macromolecules 2011, 44, 1110-1113). Similarly, Darensbourg et. al. reported that terpolymerization of propylene oxide, vinyl oxide and CO2 provided random polycarbonate copolymers of various compositions depending on the feed ratios of the epoxide monomers catalyzed by binary and bifunctional (salen) Co(III) complexes, the vinyl group introduced could be crosslinked afterwards (D. J. Darensbourg, et. al., Polymer Chemistry 2014, DOI: 10.1039/c4py01612b). Due to the high selectivity of catalysts to one kind of epoxide monomer, other strategy had to be employed to get block copolymers other than polycarbonates. Using various polymers containing hydroxyl or carboxylic group as a chain transfer agents, Lee et. al. synthesized block copolymers of PPC, and poly(ethylene oxide), polytetrahydrofuran, polycaprolactone, polystyrene, etc. respectively (A. Cyriac, et al, Macromolecules 2010, 43, 7398-7401). Alternatively, Williams's and Lu's group reported the preparation of polycarbonate block copolymer in a two-step process, the end or side hydroxyl groups due to transfer or hydrolysis of polycarbonate produced in the first step, subsequently initiate the polymerization of lactide; ABA-type and grafted polycarbonate-b-polylactide were obtained respectively (M. R. Kember, et al, Polymer Chemistry 2012, 3, 1196-1201; Y. Liu, et al, Macromolecules 2014, 47, 1269-1276). Recently, Darensbourg have demonstrated a tandem catalytic approach for the synthesis of AB diblock copolymers containing poly(styrene carbonate) and polylactide, where the end hydroxyl group of macroinitiator was generated at the end of copolymerization of styrene oxide/CO2 copolymerization (G.-P. Wu, et al, J. Am. Chem. Soc. 2012, 134, 17739-17745); in another strategy, they reported the synthesis of ABA-type PLA-PPO-PLA triblock copolymers in one pot, here, water was added along with the propylene oxide (PO)/CO2 copolymerization process as a chain-transfer reagent (D. J. Darensbourg, G. P. Wu, Angew. Chem. Int. Ed. 2013, 52, 10602-10606).
Recently, more attention has been paid to green processes and catalysts based on main group metal complexes. With efficient catalysts such as Co(III) and Cr(III), the traces of metal residues inside the resin may result in toxic, colored, degradation issues that will affect their performance and limit their applications accordingly. In contrast, aluminum, one of the earliest investigated metal as catalyst since the discovery of copolymerization of CO2 and epoxides, is earth-abundant, cheap, and biocompatible. More importantly, aluminum complexes are known to catalyze a wide range of other polymerization reactions, thus providing the possibility to expand CO2 based block copolymers other than epoxides. In fact, due to the competitive homopolymerization of epoxides catalyzed by aluminum catalysts, more work needs to be done to improve the catalytic effects. Aluminum porphyrin complex and Schiff base complexes both could catalyze alternating copolymerization of CO2 and epoxides, the catalytic efficiencies were quite low, and molar masses of obtained polycarbonates were below 10 Kg mol−1 (N. Ikpo, J. C. Flogeras, F. M. Kerton, Dalton Trans., 42, 2013, 8998-9006). As for aluminum alkoxides [(triisopropoxide (T. A. Zevaco, et. al. Green Chem., 2005, 7, 659-666); bisphenoxide (T. A. Zevaco, et. al. Catal. Today, 2006, 115, 151-161); calixarenoxide (W. Kuran, et. al. J. Macromol. Sci., Pure Appl. Chem., 1998, A35, 427-437)], these relatively simple coordination complexes, however, required high pressures, the achieved polymers were of low to moderate carbonate linkage with low molar mass. The only exception is the results reported by Kerton (N. Ikpo, et. al. Organometallics, 2012, 31, 8145-8158) that a relatively high molecular weight polymer (20.9 Kg mol−1) with 54% of carbon dioxide incorporation was achieved when amine-phenoxide was used as catalyst.
The composition of carbonate linkage in these systems could be hardly fine-tuned once the catalysts for the copolymerization of CO2 and epoxides were chosen, which then yielded for each system a fixed percentage of carbonate linkage between 100% and a few percent. The only means in each of these systems to vary the percentage of carbonate linkage would thus be to vary the pressure of CO2 or the temperature. For some purposes, polymers whose level of carbonate linkages could be easily varied may also be desirable. However, one example that allows tuning of the composition of carbonate linkage is reported by Lee et. al. who mixed two kinds of catalysts in different ratio, the propagation occurring through shuttling of the growing polymer chains between the two catalyst sites: Salen-cobalt(III) complex bearing four quaternary ammonium salts [a highly active poly(propylene carbonate) catalyst, 100% of carbonate linkage] and a double metal cyanide [DMC, a highly active poly(propylene oxide), 10% of carbonate linkage], copolymers with 10-67% of carbonates could be achieved (J. K. Varghese, et al, Polyhedron 2012, 32, 90-95).